In particle therapy, gantry systems are used to irradiate patients from different directions with particles, that is to say protons, carbon ions or oxygen ions, for example. For this purpose the gantry comprises a plurality of deflection magnets which rotate about the axis of rotation of the gantry system. Beam monitoring elements are usually arranged at a beam exit. These elements supply, for example, information concerning the site and intensity of the particle beam and are used for the output of controlled variables which notify a control system of the site and number of particles administered and of the amount of energy used. This is necessary especially in the case of radiation therapy using a scan system, since here the proton beam scans over the tissue to be irradiated in the form of what is called a pencil beam. Examples of gantry systems are known, for example, from EP 1 396 278 A2, EP 1 479 411 B1 and EP 1 402 923 A1.
In known gantry arrangements the particle beam is directed onto a stationary treatment target known as the isocenter. The irradiation angle can be freely adjusted such that a patient awaiting treatment can be irradiated from different directions in the same treatment position. Owing to the high particle energies of some 100 MeV, correspondingly large deflection magnets are required for beam guidance. These magnets are arranged on a frame and can rotate on circular paths with relatively large radii of, for example, some meters about an axis of rotation which extends through the isocenter. The problem thus arises, in particular, that the gantry system, which has been dimensioned with a view to adequate rigidity, nevertheless, depending on the angle of rotation, undergoes different displacements or distortions or deformations such that, for example, the positions of the deflection and beam-forming magnets vary. This has a disadvantageous effect on precision during guidance of the particle beam and thus on accuracy of aim and reproducibility.
The problem of the considerable structural dead weight has a heightened effect in the case of gantry systems for heavy ions (carbon ions), since here the magnets and the beam guidance elements arranged between the magnets are substantially heavier.